Unbreak build for non-ELF ports

As reported at
<https://sourceware.org/ml/gdb-patches/2017-12/msg00229.html>, this
commit:

~~~~
 commit abccd1e7b7a37385159610ca4e0bc2632a547e9a
 Author:     Jan Kratochvil <jan.kratochvil@redhat.com>
 AuthorDate: Fri Dec 8 22:44:11 2017 +0000

     Change dwarf2_initialize_objfile's return value

     dwarf2_initialize_objfile was returning boolean whether it is psymtabs
     or .gdb_index while now it needs to return also whether it is
     .debug_names.
~~~~

breaks non-ELF-target builds:

 dwarf2read.o: In function `dwarf2_initialize_objfile(objfile*)':
 /home/yao.qi/SourceCode/gnu/binutils-gdb/gdb/dwarf2read.c:6486:
 undefined reference to `elf_sym_fns_gdb_index'
 /home/yao.qi/SourceCode/gnu/binutils-gdb/gdb/dwarf2read.c:6490:
 undefined reference to `elf_sym_fns_debug_names'
 /home/yao.qi/SourceCode/gnu/binutils-gdb/gdb/dwarf2read.c:6495:
 undefined reference to `elf_sym_fns_lazy_psyms'
 collect2: error: ld returned 1 exit status
	 Makefile:1920: recipe for target 'gdb' failed

because gdb/elfread.c is not included in the gdb build unless bfd also
includes elf support.

Fix this by reverting the patch mentioned above and at the same time
re-adding .debug_names support by adding a new output parameter to
dwarf2_initialize_objfile to indicate the index variant in use.  We
can reuse the new dw_index_kind enum in dwarf2read.c for that.

gdb/ChangeLog:
2017-12-11  Pedro Alves  <palves@redhat.com>

	* defs.h (elf_sym_fns_lazy_psyms, elf_sym_fns_gdb_index)
	(elf_sym_fns_debug_names): Move to elfread.c.
	* dwarf2read.c (dwarf2_initialize_objfile): Return a boolean
	instead of a sym_fns and add 'index_kind' output parameter.  Fill
	the latter in with the index variant kind if using an index.
	(enum dw_index_kind): Moved to symfile.h.
	* elfread.c (elf_sym_fns_gdb_index, elf_sym_fns_debug_names)
	(elf_sym_fns_lazy_psyms): Move from defs.h.
	(elf_symfile_read): Adjust to new dwarf2_initialize_objfile
	interface.
	* symfile.h (enum class dw_index_kind): New, moved from
	dwarf2read.c.
	(dwarf2_initialize_objfile): Change prototype.

1 files changed, 14 insertions, 18 deletions

diff --git a/gdb/dwarf2read.c b/gdb/dwarf2read.c
index 2aeb506..b4e60a4 100644
--- a/gdb/dwarf2read.c
+++ b/gdb/dwarf2read.c
@@ -6451,11 +6451,10 @@ const struct quick_symbol_functions dwarf2_debug_names_functions =
   dw2_map_symbol_filenames
 };
 
-/* Initialize for reading DWARF for this objfile.  Return 0 if this
-   file will use psymtabs, or 1 if using the GNU index.  */
+/* See symfile.h.  */
 
-const sym_fns &
-dwarf2_initialize_objfile (struct objfile *objfile)
+bool
+dwarf2_initialize_objfile (struct objfile *objfile, dw_index_kind *index_kind)
 {
   /* If we're about to read full symbols, don't bother with the
      indices.  In this case we also don't care if some other debug
@@ -6483,16 +6482,23 @@ dwarf2_initialize_objfile (struct objfile *objfile)
       /* Return 1 so that gdb sees the "quick" functions.  However,
 	 these functions will be no-ops because we will have expanded
 	 all symtabs.  */
-      return elf_sym_fns_gdb_index;
+      *index_kind = dw_index_kind::GDB_INDEX;
+      return true;
     }
 
   if (dwarf2_read_debug_names (objfile))
-    return elf_sym_fns_debug_names;
+    {
+      *index_kind = dw_index_kind::DEBUG_NAMES;
+      return true;
+    }
 
   if (dwarf2_read_index (objfile))
-    return elf_sym_fns_gdb_index;
+    {
+      *index_kind = dw_index_kind::GDB_INDEX;
+      return true;
+    }
 
-  return elf_sym_fns_lazy_psyms;
+  return false;
 }
 
 
@@ -26799,16 +26805,6 @@ assert_file_size (FILE *file, const char *filename, size_t expected_size)
   gdb_assert (file_size == expected_size);
 }
 
-/* An index variant.  */
-enum dw_index_kind
-{
-  /* GDB's own .gdb_index format.   */
-  GDB_INDEX,
-
-  /* DWARF5 .debug_names.  */
-  DEBUG_NAMES,
-};
-
 /* Create an index file for OBJFILE in the directory DIR.  */
 
 static void